Aircraft are designed to travel through the air, i.e. the structure thereof is optimized for air as a medium. However, even aircraft have structural means which enable them to travel along the ground, referred to as the undercarriage. The undercarriage of an aircraft serves to enable the aircraft to move safely and comfortably even in contact with the ground. Moreover, the undercarriage is also used for landing or starting the aircraft, i.e. for transferring the aircraft from or to the proper medium for its movement. In particular, ground contact is reduced to a relatively small area in relation to the weight and size of the aircraft by the wheels arranged on the undercarriage, and therefore, due to poor weather conditions, for example, and the resulting poor condition of the runway, a risk to safety may arise. Thus it is conceivable, for example, that crosswinds, an engine failure or failure of reverse thrust on one side may lead to corresponding transverse forces on the undercarriage of a magnitude such that the wheels of the undercarriage lose ground adhesion. The wheels then begin to skid and maneuverability is impaired.
However, even in the case of a crosswind landing by the De-Crab method, in which the aircraft approaches the runway at a correction angle in the direction of the wind and hence the wheels of the undercarriage are not aligned in the actual direction of motion of the aircraft, it is possible, if the pilot does not steer the longitudinal axis of the aircraft in the direction of motion at the right time, just before touchdown, that the wheels may touch down on the runway at a corresponding angle and begin to skid due to the speed and weight of the aircraft, severely impairing maneuverability and therefore safety.
U.S. Pat. No. 6,722,610 B1, for example, has disclosed a steerable undercarriage for aircraft which, in the case of a crosswind landing by the De-Crab method, aligns the wheels of the undercarriage in the direction of motion of the aircraft, thus allowing the aircraft to land safely on the runway at the correction angle required by the crosswind without the pilot turning the longitudinal axis of the aircraft in the direction of motion beforehand, this being regarded as the most critical moment in such a crosswind landing. Any transverse forces on the undercarriage of the aircraft can thus be avoided.
The abovementioned patent document furthermore describes a method for increasing the braking effect after the aircraft touches down by turning the pivotably arranged wheels of the undercarriage by such an angle over the direction of motion of the vehicle that abrasion of the rubber of the wheels is increased. This is supposed to ensure that the aircraft slows down more quickly or that deceleration increases owing to the increased friction between the ground covering and the tires. However, there is the risk here that the static friction between the ground covering and the tires, which is necessary for ground contact, may change to dynamic friction, this being referred to colloquially as skidding and leading to the aircraft becoming unstable and no longer having acceptable maneuverability. Moreover, dynamic friction has the disadvantage as compared with static friction that the braking effect declines significantly since its coefficient of friction is lower than that with static friction, and hence less energy, kinetic energy, is converted by friction. Moreover, this method has the considerable disadvantage that extreme transverse forces act on the undercarriage of the aircraft, which is not designed for this, leading potentially, in the worst case, to breakage of the undercarriage. Ultimately, therefore, this method is not suitable in practice for ensuring safe and comfortable landing of an aircraft.
In the case of land vehicles too, however, e.g. cars, the transition from static friction to dynamic friction between the ground covering and the wheels is known. Such a transition occurs, for example, when a vehicle which does not have ABS is braked sharply and the wheels lock up. The braking distance of such a vehicle is significantly greater since less energy can be dissipated by dynamic friction than by means of static friction. In the case of sharp steering movements too, however, the front wheels of a car may lose adhesion and thus not steer in the desired direction but begin to skid, the result being that the vehicle maintains its previous direction of motion.